As the field of radio frequency identification (RFID) progressively advances there is an increasing need for RFID systems having a high data rate of the scale of 10 Mbit/s or above. The need for a high bit-rate RFID system is based on the fact that the power consumption and price of non-volatile memories are continually on the decrease. Increasingly more novel memory technologies are being developed which offer new perspectives in terms of the reduction of price and power consumption. The development in memory technology enables the use of mass memories of a size in the scale of mega-to-gigabytes in passive RFID tags remotely powered by a reader or another device. Current RFID air-interfaces, however, typically support data rates of up to the order of hundreds of kilobits per second and are mainly based on back-scattering and modulation of continuous wave signals. Such methods, however have some technical limitations from a performance perspective and these limitations are especially emphasised when RFID reader functions are integrated into devices with small physical dimensions.
Impulse radio based UWB provides an interesting solution for high data rate communication applications due to the available wide bandwidth. Rather than modulating a continuous carrier frequency, pulsed UWB communication is based on the transmission of short pulse signals. As illustrated in FIG. 1, the modulation can be based on the presence or absence of a pulse signal (OOK modulation), the pulse signal position (PPM) or the pulse polarity (B-PSK). Due to the short pulse signal duration, the width of the frequency spectrum is broad. In accordance with the Shannon theory, this makes it possible to achieve a higher data rate than can be achieved in traditional narrow band solutions.
Since UWB provides a wide available bandwidth, very short pulses may be transmitted such that the duration of one pulse is much lower than the symbol duration of the data. This results in a low duty-cycle of the system. As an example, if the target data rate is 10 Mbit/s the maximum symbol interval is 100 ns/symbol. The duration of UWB pulses can, however, be much lower such as 1-10 ns. Even with a pulse duration of 10 ns the duty cycle is only 10%. With shorter pulses the duty cycle is of course even lower. The low duty-cycle on air-interface can be directly utilized advantageously in the power consumption of transceiver. Depending on the duty cycle it may be necessary to keep the transceiver active only, for example, 10% of the time. To make this possible it is necessary to have good frequency and phase synchronisation between transmitting and receiving devices. In practice it is usually more straightforward to adapt the timing of the receiver to transmission than vice versa and thus the major part of the work is typically carried out at the receiving end to synchronise reception by the receiver of a transmitted pulse signals. The device initiating the communication, for example an RFID reader, typically transmits a continuous RF wave to power up the other end, for example an RFID tag, and can also serve as a common clock reference for both ends in order to ensure that both ends operate at the same frequency. Before data communication through impulse UWB is possible, phase synchronisation between the co-operating devices should be achieved.
Frequency and phase synchronisation in UWB communication present many challenges. In particular, since the transmission is pulsed rather than continuous, in cases where the transceivers of communication devices are not continuously active, i.e. the receiver detection is discontinuous, the timing of active reception periods of the receiving device should be synchronised in some way with the incoming transmitted signal from the transmitting device. Otherwise, the receiving device has a high probability of missing the incoming signal transmitted by the transmitting device. There is thus a need to synchronise the active reception periods of a receiving device with the transmission of signals from a transmitter.
The feasibility of remote powering of mass memory tags and maximum communication range requires that RFID tag power is extremely optimized and kept as low as possible. As a consequence power hungry high frequency synthesis like PLL (phase-locked loop) or DLL (Delay Locked Loop) and digital tracking systems should not be implemented in tags for sufficient frequency and phase synchronisation since otherwise the power consumption could easily grow to a level unfeasible for remote powering.
A factor that further hinders synchronisation of UWB impulse radios is the regulations set for UWB transmission spectrum. In UWB systems long spreading sequences (time-hopping codes) are required to remove spectral lines resulting from pulse repetition present in the transmitted signal. FIG. 2 illustrates, as an example, the principle of time-hopping in the time domain using On-Off-Keying modulation, which is commonly used in UWB impulse radios. The code sequences could also be efficiently used for multiple user access.
Since the benefits of time-hopping are obvious from the perspective of the transmission spectrum, it is of interest to use it in UWB impulse radio communication. However, its use results in a more time-consuming synchronisation procedure since the phase of time-hopping scheme must be known by both the transmitter and the receiver of the system before a reliable communication link can be set up. In constant pulse repetition, in practice, the maximum number of needed iteration rounds would directly be the number of possible pulse positions within one symbol which is four in the example of FIG. 2. However, when time-hopping scheme is used, the maximum required number of iteration rounds increases to the number of symbols used for time-hopping scheme multiplied by the number of time-hopping positions per symbol. As can be easily understood, the use of time-hopping makes the phase synchronisation procedure time-consuming but is essential to achieve a smooth transmission spectrum.
In short, some of the main factors that render phase synchronisation in low-power UWB impulse radio systems challenging are:
the low duty-cycle needed to keep average power consumption at a reasonable level; and
time-hopping of UWB pulses which is needed to keep the transmission spectrum smooth.
Prior art methods for finding phase synchronisation between two or more UWB devices involve searching for correlation between the incoming pulse sequence and a known reference code sequence by sliding and fine-tuning the timing of reference sequence. As soon as the phase between the received signal and the reference code is correct, a correlation peak emerges. This method is called serial search. A parallel search is an alternative method to speed up the synchronisation process. Nevertheless, parallel search requires complex circuit receiver because each branch is duplicated.
An example diagram of such a method is illustrated in FIG. 3. In this example the reader transmits time-hopping synchronisation sequences periodically. The counterpart device, a tag in this case, listens to the synchronisation sequences and correlates the input data with the reference sequence. After each correlation round it may pause so that the incoming sequence will finally match with the reference sequence. As soon as the correlation is high enough, the tag may prepare itself for sending information to the reader during the predefined time slot. However, since the time-hopping sequences must be relatively long to achieve a smooth transmission spectrum, finding the correct phase between the sequences is time consuming especially when the duty-cycle of receivers at both ends of the system are low due to optimization of power consumption, as described above.
Another weakness of the prior art serial search synchronisation method is that the complexity is at the receiver side and not in the emitter. For this reason, the use of the prior art method necessitates that, in case of passive memory tags, the tag should be capable of doing correlation between incoming code sequence and the known reference sequence. The other alternative is of course that the tag would be the emitter of the synchronisation sequence which decreases the complexity in the tag. When entering the field of communication the tag would send the synchronisation pulse on a regulator time base, and the reader would have to synchronise itself on the tag synchronisation pulse. This behaviour is the well known “tag talk first” principle. However, a drawback of such an arrangement is collision management since each tag in or entering the field contributes to channel saturation.
Some improvements to the traditional serial search method with spread-spectrum sequences are proposed in US2006/0093077 in an attempt to address some of the aforementioned problems. This document describes a method of synchronisation based on a cross correlation between an input signal and a template pulse train. However, the system does not address all of the above problems.
An embodiment of the serial search synchronisation approach is based on super-regenerative architecture. This architecture was widely used in wartime, in pulse responders for radar identification. In such a system, an interrogator sends an interrogating pulse to a transponder to be identified. The fundamental theory of super-regeneration was established during this period. More recently, the application of the super-regenerative receiver has extended to narrow-band systems in which reduced cost and low power consumption are required. The technology has very recently been extended to Ultra Wideband pulse communication which implies a novel approach with regard to super-regenerative architecture optimization. In this technology, precise timing is required for synchronization of the bi-directional communication between transceivers. There is thus a requirement for a power efficiency method to cope with synchronization issues in UWB communication based on super-regenerative receiver.
A problem that may be encountered using super-regenerative technology is the generation of signals by a super-regenerator in a transceiver even when there is no incoming transmission pulse from a co-operating transceiver, for example in response to an incoming signal from another transceiver or originating from noise or other interference. Difficulties arise in distinguishing an intended incoming pulse from a co-operating transceiver and an incoming signal resulting from other effects.
The European patent application EP 1 503 513 A1 discloses a method for identifying the beginning of an Ultra Wideband pulse sequence.